The CGG repeat in the FMR1 gene causes the Fragile X Syndrome by massive expansion and extinction of expression of FMR1. We have created mice carrying CGG repeats within a minigene containing the first exon of the FMR1 gene placed downstream of the minimal FMR1 promoter. These minigenes contain 9,57 and 105 uninterrupted repeats. Mice carrying these minigenes were established and the repeats were found to be completely stable through multiple passages through the mouse germline.